1. Field of the Invention
The present invention relates to an image encoding apparatus for encoding a binary image.
2. Related Background Art
In a facsimile apparatus as a typical still image communication apparatus, a method of encoding an image in accordance with MH or MR coding by sequentially scanning the image is employed. In this method, in order to recognize the entire image, all the image data must be transmitted, resulting in a long transmission time. Therefore, it is difficult to apply this method to an image communication service such as an image data base service, a videotex, or the like, in which an image must be quickly judged.
Unlike the method employed in the facsimile apparatus, a sequential reproducing/encoding method in which rough image data is transmitted before the entire image is transmitted, and additional data is then transmitted to generate detailed image data is proposed in, e.g., "Sequential Reproducing/Encoding Method of Facsimile Signal Suitable for Conversation Type Image Communication", Endo and Yamazaki, (Transaction of the institute of electronics, information and communication engineers, (B) J67-B.12, pp. 1462-1469 (1984)).
In a so-called progressive encoding method for generating images having different resolutions and separately encoding and transmitting the images having the different resolutions, in order to obtain a low-resolution image, an input image is subjected to sampling processing at a predetermined rate.
However, since initial image data is generated by sampling pixels at specific intervals, encoding efficiency of a normal document image is good while effective image data cannot be transmitted in a first stage depending on the type of image.
Especially, if an image which is dither-processed for pseudo halftone reproduction is simply sampled, an image is omitted depending on a sampling period. When an encoded image is reproduced, an original image cannot be faithfully reproduced.